In the manufacture of insulated strip conductor for transformers, bare metal strips are coated with a resin, the resin is cured, and the coated strips are wound into supply coils. Coated strips are typically made by passing bare aluminum strip through a solvent-based enamel, then into a heating tower to evaporate the solvent and cure the enamel. Typically, about three passes are required to obtain a sufficiently thick build on the strips to provide adequate insulation. This process is not as economical as it might be because heat is needed to evaporate the solvent, and the solvent must then be collected or incinerated to prevent environmental pollution.
A 100% solids enamel can also be used for coating the strips. Such enamels can be prepared by using a monomer instead of a solvent. An oligomer is dissolved in the monomer and the monomer then reacts with the oligomer during cure. While this eliminates the solvent, these enamels cure slowly and some of the monomer is still lost to evaporation.
Another difficulty in formulating suitable compositions for this purpose, is that it is difficult to find polymers which are stable in the hot oil environment of transformers. Oil-filled transformers contain paper insulation which releases water when the transformer is in operation. The hot water then hydrolyzes the polymer coated on the strips, especially if they are condensation polymers. The problem is aggravated by the presence of acid. The acid degrades the paper, releasing still more acid, a chain reaction which can rapidly destroy the paper insulation. An acid also accelerates an aqueous attack against a condensation polymer. Acid acceptors are usually added to neutralize the acid, but if the pH in any area of the transformer is too high or too low, the polymer in that area may be attacked by hydrolysis.